1) Field of the Invention
The present invention relates to devices used in the optical communications.
2) Description of the Related Art
FIG. 11 is an external view of an optical transmitter 10 and an optical receiver 20 in a conventional optical communication system.
The optical transmitter 10 transmits a laser beam via an optical fiber 14. The optical transmitter 10 includes a case 11. The case 11 houses a laser beam generator (not shown) that generates a laser beam, a first supporting (not shown) member that supports the laser beam generator, a lens (not shown) that concentrates the laser beam from the laser beam generator, a second supporting member (not shown) that supports the lens, a member (not shown) via which the first support member and the second support member are attached to each other, and a cooler (not shown) that cools down heat generated in the laser beam generator and the lens. The member (hereinafter, “high weldability member”) via which the first support member and the second support member are attached to each other is easy to weld.
The high weldability member is made of a material whose weldability by YAG welding is high (therefore robust welding may be made) such as KOVAR®,which is a Fe-Co-Ni alloy. The cooler is of a type that performs cooling, for example, based on Peltier effect. In the Peltier effect, when weak current flows through a joint of two metal plates, heat transfers from one plate to the other plate. A driver 12 is arranged outside of the case 11. The driver 12 drives and controls the laser beam generator via a lead wire having a line length. An end of an optical fiber 14 is inserted into and fixed to an optical fiber tube 13.
As the laser beam generator is driven under the control of the driver 12. a laser beam is ejected from the laser beam generator. This laser beam is concentrated by the lens. The concentrated laser beam enters an incident surface of the optical fiber 14, and goes out through the optical fiber 14.
Meanwhile, heat generated in the laser beam generator and the lens is conveyed to the cooler via the first support member, the second support member, and the high weldability member. As a result, the temperature in the case 11 is maintained below a temperature guarantees proper functioning of the device.
Because large amount of heat is generated in the laser beam generator and the lens, a cooler with a high performance is used. A high performance cooler generally has a large size. As a consequence, the overall size of the optical transmitter 10 exceeds 30 millimeters.
On the other hand, the optical receiver 20 receives a laser beam via an optical fiber 23. The optical receiver 20 includes a case 21. The case 21 houses a lens (not shown) that concentrates the laser beam from the optical fiber 23, a photo diode (not shown) that receives the laser beam and converts the laser beam into electric signals, and a preamplifier (not shown) that amplifies the electric signals. An end of the optical fiber 23 is inserted into and fixed to an optical fiber tube 22.
A laser beam comes in through the optical fiber 23 and ejected from an outgoing surface of the optical fiber 23. The laser beam is concentrated by the lens, and converted into electric signals by the photo diode. The electric signals are amplified by the preamplifier.
A conventional technology has been disclosed in Japanese Patent Application Laid-Open Publication No. 2002-82261.
Recently there has been a high demand that the optical communication system be high speed (of the order of several tens of gigabits per second) and compact. See, for example, XFP (10 Gigabit Small Form Factor Pluggable) standard.
According to the XFP standard, the distance between the optical axis of the optical transmitter and that of the optical receiver be 6 millimeters. However, in the conventional optical transmitter and the optical receiver, it is not possible to achieve such compactness. Precisely, due to the large-sized cooler, as mentioned above, the overall size cannot be made smaller.
Moreover, because the driver 12 is positioned outside of the optical transmitter 10, a line length of the lead line between the driver 12 and the laser beam generator becomes longer. As a result, high frequency properties of the optical transmitter deteriorate.
Furthermore, because the photo diode and the preamplifier are arranged inside the case 21 of the optical receiver 20, the operations for exchanging the photo diode and the preamplifier become cumbersome. Moreover, testing of the device becomes difficult. Moreover, design of the optical receiver cannot be changed so easily.